Cathepsin S (EC 3.4.22.27) is a cysteine protease of the papain family found primarily in lysosomes (Bromme, D.; McGrath, M. E. High Level Expression and Crystallization of Recombinant Human Cathepsin S. Protein Science 1996, 5, 789-791).
The role of cathepsin S in the immune response is anticipated by its tissue distribution: cathepsin S is found primarily in lymphatic tissues, lymph nodes, the spleen, B lymphocytes, and macrophages (Kirschke, H. Chapter 211. Cathepsin S. In Handbook of Proteolytic Enzymes. Barrett, A. J.; Rawlings, N. D.; Woessner, J. F., Eds. San Diego: Academic Press, 1998. pp. 621-624.). Cathepsin S inhibitors have been shown in animal models to modulate antigen presentation and are effective in an animal model of asthma (Riese, R. J.; Mitchell, R. N.; Villadangos, J. A.; Shi, G.-P.; Palmer, J. T.; Karp, E. R.; De Sanctis, G. T.; Ploegh, H. L.; Chapman, H. A. Cathepsin S Activity Regulates Antigen Presentation and Immunity. J. Clin. Invest. 1998, 101, 2351-2363 and Shi, G.-P.; Villadangos, J. A.; Dranoff, G.; Small, C.; Gu, L.; Haley, K. J.; Riese, R.; Ploegh, H. L.; Chapman, H. A. Cathepsin S Required for Normal MHC Class II Peptide Loading and Germinal Center Development. Immunity 1999, 10, 197-206.).
Mice in which the gene encoding cathepsin S has been knocked out are less susceptible to collagen-induced arthritis and their immune systems have an impaired ability to respond to antigens (Nakagawa, T. Y.; Brissette, W. H.; Lira, P. D.; Griffiths, R. J.; Petrushova, N.; Stock, J.; McNeish, J. D.; Eastman, S. E.; Howard, E. D.; Clarke, S. R. M.; Rosloniec, E. F.; Elliott, E. A.; Rudensky, A. Y. Impaired Invariant Chain Degradation and Antigen Presentation and Diminished Collagen-Induced Arthritis in Cathepsin S Null Mice. Immunity 1999, 10, 207-217).
These data demonstrate that compounds that inhibit the proteolytic activity of human cathepsin S should find utility in the treatment of chronic autoimmune diseases including, but not limited to, lupus, rheumatoid arthritis, and asthma; and have potential utility in modulating the immune response to tissue transplantation.
There are a number of cathepsin S inhibitors reported in the literature. The most important patents are listed below.
Certain dipeptidyl nitriles are claimed by Novartis as cathepsin S inhibitors in: Altmann, et. al. WO-99/24460.
Dipeptidyl vinyl sulfones are claimed by Arris (now Axys) as cysteine protease (including cathepsin S) inhibitors in: Palmer, et. al. U.S. Pat. No. 5,976,858.
Certain peptidyl sulfonamides are claimed by Arris/Axys as cysteine protease (including cathepsin S) inhibitors in: Palmer, et. al. U.S. Pat. No. 5,776,718 (assigned to Arris, now Axys) & Klaus, et. al. U.S. Pat. No. 6,030,946 (assigned to Axys).
Compounds somewhat similar to those of the present invention are described in the following references.
Winters, et. al. (Winters, G.; Sala, A.; Barone, D.; Baldoli, E. J. Med. Chem. 1985, 28, 934-940; Singh, P.; Sharma, R. C. Quant. Struct.-Act. Relat. 1990, 9, 29-32; Winters, G.; Sala, A.; Barone, D. in U.S. Pat. No. 4,500,525 (1985)) have described bicyclic pyrazoles of the type shown below. R never contains a heterocyclic ring and no protease inhibitor activity is ascribed to these molecules; they are described as α1-adrenergic receptor modulators. 
Shutske, et. al. claim the bicylic pyrazoles below. The pyridine ring is aromatic in their system (Shutske, G. M.; Kapples, K. J.; Tomer, J. D. U.S. Pat. No. 5,264,576 (1993)). Although reference is made to R being a linker to a heterocycle, the claims specify only R=hydrogen. The compounds are referred to as serotonin reuptake inhibitors. 
The compound 2-[4-[4-(3-methyl-5-phenyl-1H-pyrazol-1-yl)butyl]-1-piperazinyl]-pyrimidine is known from EP-382637, which describes pyrimidines having anxiolytic properties. This compound and analogs are further described in EP-502786 as cardiovascular and central nervous system agents. Pharmaceutical formulations with such compounds are disclosed in EP-655248 for use in the treatment of gastric secreation and as anti-ulcer agents. WO-9721439 describes medicaments with such compounds for treating obsessive-compulsive disorders, sleep apnea, sexual dysfunctions, emesis and motion sickness.
The compounds 5-methyl-3-phenyl-1-[4-(4-phenyl-1-piperazinyl)butyl]-1H-indazole and 5-bromo-3-(2-chlorophenyl)-1-[4-(4-phenyl-1-piperazinyl)butyl]-1H-indazole, in particular the hydrochloride salts thereof, are known from WO-9852940 and CA 122:314528, where these and similar compounds are described as kinase inhibitors in the former reference and possessing affinity for benzodiazepine receptors in the latter reference.